1. Field of the Invention
The present invention relates to an automatic gain control (AGC) circuit of a mobile communication terminal, and particularly, relates to a circuit for compensating temperature of an AGC circuit.
2. Background of the Related Art
Generally, in a mobile communication system, a terminal and a base station perform gain control to prevent a receiving channel from being saturated when a receiving signal is strongly applied. There are many ways of designing a circuit for gain control. For example, there is a method of using an AGC amplifier to perform gain control as a low noise amplifier (LNA) after a duplexer. Further, there are methods of using the AGC amplifier after the LNA and of using an attenuator at the front end of the LNA.
FIG. 1 shows a related art AGC circuit using a PIN attenuator. The related art AGC circuit includes a duplexer 10 for separating radio frequency (RF) signals received through an antenna, and a PIN diode attenuator 20 for attenuating the RF signals, which were separated by the duplexer 10. Additionally, the circuit includes a LNA 30 for performing low noise amplification on the output signal of the PIN diode attenuator 20 and an attenuation control unit 40 for outputting an attenuation control signal to the PIN diode attenuator 20 by sensing the RF signal RFout, which was amplified and outputted by the LNA 30.
The PIN diode attenuator 20 includes capacitors C1 and C2, which are serially connected between the duplexer 10 and LNA 30, and a resistor R1 and a PIN diode PIN1, which are serially connected between an output terminal of an operational (OP) amplifier 42 and the capacitor C2. The PIN diode attenuator 20 further includes a PIN diode PIN2, resistor R2, and capacitor C3, which are connected between the capacitor C1 and a ground in parallel. An inductor (L2) bypasses the output signal of the second PIN diode PIN2 to the ground.
The attenuation control unit 40 includes a gain control unit 41 which generates a gain control signal AGC_cont by sensing the RF signal RFout amplified by the LNA 30, and an operational (OP) amplifier 42 for outputting the attenuation control signal by amplifying the gain control signal AGC_cont outputted from the gain control unit 41. Subsequently, the inductor(L1) blocks an alternating current component that flows into the OP amp 42.
The operation of the related art AGC circuit with the above composition is that the duplexer 10 receives a radio frequency (RF) signal through an antenna and outputs the received RF signal to the LNA 30 through the capacitors C1 and C2. The LNA 30 amplifies the RF signal outputted from the duplexer 10 according to the gain control signal AGC_cont. Because the RF signal is received through a multiplex passage, the received RF signal becomes either higher or lower due to the signal overlapping of the multiplex passages. If an RF signal having a non-uniform receiving level is amplified in the LNA 30, the level of the received RF signal especially needs to be attenuated properly. When the level of change of the RF signal is higher, the effect of the possible error is larger.
The gain control unit 41 of the attenuation control unit 40 compares the level of the RF signal RFout, which is outputted from the LNA 30, with a reference level and outputs the gain control signal AGC_cont to the OP amplifier 42. The OP amplifier 42 then outputs an attenuation control signal according to the gain control signal AGC_cont to uniformly maintain the level of the RF signal, which is inputted to the LNA 30.
Further, the PIN diode attenuator 20 attenuates the RF signal, which is later inputted to the LNA 30, according to the attenuation control signal that was outputted from the attenuation control unit 40. The attenuation amount of the RF signal is determined by the voltage level of the attenuation control signal, which is inputted to the PIN diodes PIN1 and PIN2, as shown in FIG. 2A. Hence, when the voltage level of the attenuation control signal, which is outputted from the OP amplifier 42 by the gain control signal AGC_cont is increased, the amount of the current, which flows to the PIN diodes PIN1 and PIN2, is also increased. When the amount of the current is increased, the internal resistance of the PIN diodes PIN1 and PIN2 is decreased. Therefore, the attenuation amount of the RF signal is increased.
If the gain control signal AGC_cont is changed by 1.6V, an attenuation of 12.5 dB is generated. This amount means that 0.7 dB of attenuation is generated per 1 mV. In addition, the extent of the attenuation amount can be adjusted by changing the value of resistors R1 and R2. The attenuation of the related art is designed to be as much as the gain of the LNA 30 when the maximum gain control signal AGC_cont is maximized.
When temperature of the circumstance is changed, the internal resistance of the PIN diode of the attenuator is changed accordingly. Namely, the resistance of the PIN diodes PIN1 and PIN2 is increased at high temperatures and decreased at low temperatures. As a result, when the temperature is changed, the voltage between the PIN diodes PIN1 and PIN2 is changed; that is, the amount of current is changed. As shown in FIG. 2B, the voltage between node A and ground is changed at an approximate rate of 5.2 mV/° C. Thus, the capacitor C3 has a function of removing a DC component.
Therefore, when temperature changes from +60° C. to −30° C., the voltage between node A and ground changes 0.47V due to the change in current flowing in the PIN diodes PIN1 and PIN2, and the current value is converted as attenuation amount of 3.2 dB. Specifically, 3.2 dB of attenuation difference can be generated between the maximum and minimum temperatures.
As described above, the related art AGC circuit has various problems. For example, because internal resistance of the PIN diode is changed by the surrounding temperature of the AGC circuit, the attenuation amount of the PIN diode attenuator is changed. Accordingly, gain control of the RF signal cannot be achieved properly.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.